1. Field of the Invention
The present invention relates to image sensors. More particularly, the present invention relates to a shared-pixel-type image sensor with enhanced light-receiving efficiency and a method of fabricating the same.
2. Description of the Related Art
Image sensors convert optical images into electrical signals. The demands for image sensors with enhanced performance are increasing in various fields such as digital cameras, camcorders, personal communication systems (PCSs), game devices, security cameras, and micro-cameras for medical use.
Complementary metal oxide semiconductor (CMOS) image sensors can be driven using a simple digital driving method and can be output using various scanning methods. In CMOS image sensors, signal-processing circuits can be integrated onto a single chip. Thus, the size of a product including such a CMOS image sensor can be reduced compared with charge-coupled device (CCD) image sensors. In addition, since CMOS image sensors are compatible with CMOS processing technology, manufacturing costs can be reduced. Due to their low power consumption, CMOS image sensors can be applied in mobile products with limited battery capacity. As pixel integration (size reduction) is improved to achieve increased resolution, the area occupied by the photoelectric conversion element in each unit pixel is also reduced. The area occupied by reading elements that read optical signals incident on the photoelectric conversion elements, formed in the peripheral area, also decreases. Products including smaller photoelectric conversion elements may have lower sensitivity and less saturation of signals. An active pixel sensor array with a plurality of photoelectric conversion elements sharing (multiplexing) a smaller number of reading elements (e.g., pass-transistors, lines, sensors) has been proposed. Since the photoelectric conversion elements, which function as light-receiving units, share the reading elements, the area of each photoelectric conversion element can be maximized, thereby increasing light-receiving efficiency. Thus, a shared-pixel-type image sensor, in which two or four photoelectric conversion elements adjacent to each other in a direction share one set of reading elements, has been proposed.
In a shared-pixel-type image sensor, some of the reading elements (pass-transistors) are formed in a different active region isolated from an active region in which a photoelectric conversion element is formed. In addition, the shared-pixel-type image sensor requires a wire for connecting a floating diffusion region to a drive element. The floating diffusion region and the drive element are formed in different active regions of the shared-pixel-type image sensor. Accordingly, a conductor (e.g., metal wire) for connecting between the floating diffusion region and the drive element must be formed in the shared-pixel-type image sensor.
When wires required in a shared-pixel-type image sensor are designed, an exposed area of a top surface of each photoelectric conversion element is maximized in order to increase a fill factor. However, as the size of a device is reduced while the number of wires required increases, the exposed area between metal wires is reduced, reducing the fill factor. Therefore, it is desirable to increase the exposed area between the metal wires.